1. Field of the Invention
The invention relates to a system for reducing the coherence of a wavefront-emitting laser radiation, in particular for a projection objective in semiconductor lithography, a first partial beam of the laser beam impinging on a surface of a resonator body being partially reflected and a second partial beam of said laser beam entering the resonator body and emerging from the resonator body again after a plurality of total reflections at least approximately in the region of the entrance location and being forwarded with the first partial beam jointly to an illumination plane. The invention also relates to a projection exposure apparatus with a laser as a light source, an illumination system and a projection objective.
2. Description of the Related Art
In the case of projection objectives in semiconductor lithography, it is necessary to illuminate the mask, also called reticle, as homogeneously as possible by means of an illumination system. In this case, however, there is a problem when using pulsed laser light in terms of high temporal coherence of the laser radiation, as a result of which the homogeneity is disrupted by speckle, that is to say differences in bright/dark. It has already been proposed, therefore, to use the arrangement of a resonator body in the form of a prism having three corners to split the laser beam when it impinges on the resonator body, to be precise into a partially reflected partial beam and a second partial beam, which enters the resonator body, emerges from the resonator body again after corresponding total reflections in the region of the entrance location and is subsequently recombined with the reflected partial beam. In this way, the laser pulses are practically “chopped” into a plurality of partial beams which arrive temporally successively at the illumination plane, e.g. the reticle plane in the case of use in semiconductor lithography. The purpose in this case is to make the time interval between two pulses so long that it is longer than the so-called temporal coherence of the laser radiation. This means that the beam bundles are no longer capable of interference, that is to say that they can no longer form any interference. This measure is intended to result in an improvement in the illumination homogeneity.
With regard to the prior art, reference is made to EP 1 107 039 A2, U.S. Pat. No. 6,238,063 B1 and the Patent Abstract of Japan 01198759A.